The specific aim of this project is to test the hypothesis that an endogenous neural rhythm with a period of 19.2-20.0 hr may be involved in the regulation of LH secretion in the rat. Evidence from a variety of recent studies indicates that interaction of such a component with a photoperiod-entrained neural component may act as a pacemaker for the ovulatory LH surge. Measures of LH in serum will be made in rats at 3 hr intervals throughout the normal estrous cycle, in ovariectomized animals and in animals exposed to continuous illumination. Silastic implants of estrogen will be used in an effort to visualize and/or amplify the response. The long-term objective of this study is to determine the extent to which such rhythms may play a role in regulating the timing of seasonal reproductive activity, reproductive senescence, and differences in the neuroendocrine control of reproduction between males and females.